Incorporation of rare earth elements in the shell of freshwater mussels as an indicator for their bioavailability from polluted river water

High-technology metals — such as the rare earth elements (REE) — have become emerging contaminants in the aquatic environment. Due to their low natural background concentration and their unique element signature, anthropogenic input can be followed through the waste water discharge into the rivers to the coastal sea and can even be used as a tracer for the contribution of surface water to drinking water.

Despite the significant pollution detected during the last decade in river and lake water worlwide, surprisingly little is known about the environmental behavior of these metals and their compounds. The Rhine River and the Weser River in Germany are two prime examples of rivers that are subjected to anthropogenic REE input from different sources.  While both rivers carry significant loads of anthropogenic Gd, originating from contrast agents used for magnetic resonance imaging, the Rhine River also carries large amounts of anthropogenic La and lately Sm which are discharged into the river from an industrial point source.

Researchers from the Jacobs University Bremen in Germany now have assessed the bioavailability of these REE elements by analyzing the shells of  freshwater mussels. Mussels have been used for more than 50 years for monitoring the contamination of the aquatic environment. While most researchers have used contaminant concentrations in the soft tissue, the concentration in their shells have also be employed for biomonitoring.    
It has previously been shown that the REE distribution in marine mussel shells provides information on the REE distribution in the water in these mussels' respective habitats.

The researchers observed that the anthropogenic input of Gd to both rivers, resulting from its use as a contrast agent in medical imaging, and detectable in the river water by the so called Gd-anomaly in the REE concentration pattern, cannot be observed in the mussel shell. This indicates that the speciations of geogenic and anthropogenic Gd in the river water differ from each other and that the geogenic, but not the anthropogenic Gd is incorporated into the shells.  

While the anthropogenic Gd input is diffuse and therefore shows up in a related Gd-anomaly over the whole length of the rivers, high amounts of La and Sm realeased from an industrial production site of fluid catalytic cracking catalysts, only show up as a related anomaly downstream of the point source. Interestingly, all shells sampled at sites downstream of this industrial point source show positive La and Sm anomalies, revealing that these anthropogenic REE are bioavailable.